1. Field of the Invention
This invention relates to video signal reproducing apparatus and, more particularly, to an apparatus for reproducing a video signal from a tape-shaped recording medium by using a rotary head.
2. Description of the Related Art
Systems for recording video signals on tape-shaped recording mediums include a digital video tape recorder (VTR) in which video signals for one picture are separated into a plurality of blocks and are encoded with respect to each block unit, and data or codes such as sync codes, block ID (index) data, and error detection/correction parity codes are added to the encoded signals to form data blocks which are recorded on a magnetic tape by a rotary head in a diagonal recording manner. Data blocks for one picture are recorded in a plurality of tracks.
At the time of reproduction, data blocks in the respective recording tracks are converted into electrical signals by the rotary head, and errors in the video data are detected and corrected based on the parities in the data blocks, so that the original video signal is restored based on the sync codes and the block ID (address) data of the data blocks. FIG. 1 is a block diagram showing the basic construction of a reproduction system of an ordinary digital VTR. Sync codes are extracted by a reproduction processing circuit 10 from reproduction data reproduced by a rotary head H from a video tape T used as a recording medium. Video data, address data and other kinds of added data are extracted based on the sync codes. The series of data items thereby obtained are supplied to an error discrimination/correction circuit 12, and code errors are corrected by this circuit. The data is thereafter written in a frame memory 14. Write addresses used for this writing are determined in accordance with the address data separated in the reproduction processing circuit 10. The data stored in the frame memory 14 is read out based on read address data supplied from a read address generating circuit 16 and is supplied to an output terminal 18.
A capstan control circuit 8 drives a capstan 6 in accordance with a reproduction instruction from a system controller 4 so that the tape T is transported at the same speed as the speed maintained at the time of recording. The capstan control circuit 8 also effects tracking control in a well-known manner in order to make the head H accurately trace each track. Methods for this tracking control are known, including one in which a control signal recorded on a tape end is used, and one in which a pilot signal recorded in each track is used.
The realization of a search reproduction function of the digital VTR is considered herein whose function corresponds to that of an analog VTR for recording a video signal on a video tape in an analog recording manner. The search function is a function for making the magnetic tape travel at a speed higher than the speed set for recording or ordinary reproduction to enable the contents of recording on the magnetic tape to be observed in a short time. In the case of an analog VTR, while the rotary magnetic head moves across a plurality of tracks, part of a signal is reproduced from each track, and reproduced parts are combined to form and display one complete picture.
If the digital VTR performs the same reproduction operation as this analog VTR, signals obtained from the respective tracks by moving the rotary magnetic head across the tracks are not always based on the above-mentioned data block unit. There is, therefore, a strong possibility of failure to completely obtain data for each data block and, hence, failure to perform the desired error correction, resulting in a considerable increase in the error rate or an inability to correct error. That is, in a case where, in the digital VTR, the rotary magnetic head and the magnetic tape are operated for search reproduction in the same manner as the analog VTR, there are many data blocks which cannot be completely restored; the number of data blocks completely restored is very small. Even though interpolation is effected between frames with respect to data blocks which cannot be restored, the extent of divergence from data on adjacent picture elements with respect to time is very large and the reproduced image is very poor.